It is well known that operation of an AC induction motor at substantially less than its full rated load becomes more and more inefficient in use of electrical energy as the power factor of the load, which varies inversely with the load, increases. Numerous methods and devices for reducing energy consumption of AC induction motors have been developed or proposed. One class of such devices and methods uses a measure of the power factor of the load, e.g. an AC induction motor, to generate a control signal used for adjusting the power delivered to the motor. In order to maintain sufficient rotor xe2x80x9cslipxe2x80x9d for operation with a relatively low power factor and best efficiency, the control signal is adjusted to reduce the average power applied to the motor during light loading.
Various problems occur in the aforementioned prior art devices and methods, particularly when applied to controlling power delivered to AC induction motors which drive pumping units used to extract oil from underground deposits. Such pumping units are alternately loaded by the pumping rods and the opposing counter weights twice each pumping cycle. Moreover, twice each cycle the two opposing loads balance and the motor is thus unloaded twice each cycle. The constantly changing load between peak minimum and maximum values creates severe control difficulties for power factor control systems and methods which must continuously adjust the power delivery to maintain optimum efficiency and economy.
Some of the operational problems with prior art include: (1) systems and methods which, instead of relying on an accurate time base use counters based on imprecise time bases or are subject to drift with temperature, voltage or load or, which are in turn subject to interruption by external signals or interference, typically resulting in inefficient or inappropriate control actions; (2) systems which are susceptible to severe back-EMF effects or other electromagnetic interference generated in the motor or in the controller itself which may substantially interfere with accurate power factor sensing and control or an inability to process sensor parameters or generate unambiguous control signals because of the high interference levels present; (3) systems and methods which only work well when the motors connected to the controller are in good condition, are correctly wired to the power source and/or the controller or do not have significant phase winding irregularities imbalances or mechanically unbalanced loads which typically results in an inability to properly compensate or adjust the power factor or, in some cases, motor failure; (4) systems which must be manually adjusted to accommodate individual application conditions and thus are labor intensive, costly, and may fail to provide optimum adjustment because limited range of adjustment; (5) setting desired power factor parameters by hand or setting average power factors which once set, are fixed and, at best, only approximate the potential efficiency improvement desired from the system; and (6) systems or methods which require complex control circuitry or modification to the motors in order to provide efficient control which also tends to increase the cost of manufacturing, installing or using the systems.
In accordance with present invention a controller is provided which dynamically matches energy use by an AC induction motor to the load on the motor. First and second SCRs, each having a respective gate, are connected in parallel with each other in opposing polarities between a first node and a second node for each phase of the applied AC voltage. The first node is connected to a source of the applied voltage The second node is connected to at least one winding of the motor. A trigger generator is coupled to the respective gates of the SCRs, to the applied voltage and to the motor winding for controlling each of the first and second SCRs responsive to the respective timing of sensed zero crossing events of the AC voltage and the AC current in the motor winding corresponding to the applied AC voltage. Further, the first and second SCRs are alternately triggered into a conductive state during each alternation of the applied AC voltage and are alternately inhibited from the conductive state for an interval in time proportional to a measured difference in time between the time the AC voltage across the motor winding passes through a first zero-crossing and the time the corresponding AC current in the motor winding passes through a second zero-crossing, wherein the measured difference is determined as the time difference between successive first and second interrupts are coupled to the trigger generator and compared with a continuously running time base.
In one embodiment, the gate drive for the SCRs coupled between the first and second nodes is selectively opened and closed by the control signal provided by the trigger generator.
In another embodiment, the trigger generator comprises a first input for sensing the AC voltage applied to the motor winding; a second input for sensing the AC current in the motor winding corresponding to the AC voltage applied to the motor winding; an output for providing a trigger control signal for controlling each of the first and second SCRs; and a control device including a continuously running time base that is responsive to the first and second interrupts corresponding respectively to the first and second inputs for generating the trigger control signal.
In a preferred embodiment, the control device comprises a measurement system for measuring the elapsed time between the first and second interrupts occurring in a select phase of the applied AC voltage and for calculating the product of the elapsed time and a predetermined factor. The control device includes the capability for generating a trigger control signal, within a first time interval following the zero-crossing of the current in the motor during the select phase of the AC voltage, wherein the trigger control signal has a duration substantially equal to the product of the elapsed time and the predetermined factor; and wherein the first and second gate drive circuits are disabled for the duration of the trigger control signal. Other features and advantages of the invention will become more apparent from the following detailed description taken in connection with the appended claims and attached drawing in which: